The added security benefit of server-side hashing is the same as if plain text passwords are sent, to prevent knowledge of the authentication secret if the data

The added security benefit of server-side hashing is the same as if plain text passwords are sent, to prevent knowledge of the authentication secret if the database contents are disclosed to malicious third parties. The client side hash of the password is only to ensure that a fixed length secret is sent and subsequently processed, to avoid DoS attacks on the server.